Tubular joints are routinely used in the oil and gas industry to connect sections of conduits together to form various types of flow strings; e.g. tubing strings, casing strings, drill strings, etc. Typical of such a joint is one which is comprised of a pin member (male threads) which is welded or otherwise affixed to one end of a conduit section and a box member (female threads) similarly affixed to the other end thereof. The pin member of one section is threaded into the box member on another section and is tightened to a prescribed torque to form a fluid-tight connection between the conduits.
Unfortunately, when a threaded, shouldered connection such as a standard tubular joint is tightly made up, there is an inherent tendency for the connection to anti-rotate and loosen itself. This tendency is a function of the lead angle of the thread and is normally counteracted by the frictional forces between both the treads and the abutting shoulders of the joint. These frictional forces are considered to result from the microscopic engagement of peaks and valleys of one surface against the peaks and valleys of the other and are normally adequate to prevent unthreading of the connection, especially where the string of conduit is used in a static environment, e.g. a well bore. However, where the conduit string experiences dynamic forces over a prolonged period, a joint may work back and forth to thereby unthread and loosen to the point where an undesirable, if not disastrous, leak may occur.
For example, a critical consideration in the production of fluid hydrocarbons from marine deposits lies in providing a fluid communication system from the marine bottom to the surface after production has been established. Such a system, commonly called a production riser, usually includes multiple conduits strings through which various produced fluids are transported to and from the surface and include oil and gas production lines, service, and hydraulic control lines.
When these conduit strings which extend vertically through several hundreds of feet of water are subjected to substantial yaw moments (i.e. moments at 90.degree. to conduit centerline), microscopic separation and slippage between the frictional surfaces of the joint can occur. As this slippage is repeated over and over again for prolonged periods of operation, eventually the joint can be loosened by left-hand or anti-rotational moments well below the original make-up torque. Further, the joints in the tubular strings also are subjected to substantial cyclic tensile, axial moments and twisting moments due to the movements in the water body in which the conduit strings are positioned; any of which may result in fully reversing forces which tend to loosen or unscrew the joint.
As an example, a compliant marine riser having a rigid section 2200 feet long and having 13 separate conduit strings (each made up of standard 45 foot lengths of conduit) will have a total of 637 tubular joint connections. If the probability of only one of these joints unthreading can be assumed to one in 100 over a typical 20 year expected life of the riser, the actual probability that at least one joint will so fail is calculated as a 99.8% certainty. It can be seen that this high probability of failure and the resulting likelihood of a leak developing therefrom during the expected life of riser, dictate that steps must be taken to insure that a joint used in such enviroments can not be accidentaly unthreaded even when exposed to hostile conditions.